Supposed comparison of Mobile HF Antennas in November QST
"Roy Lewallen" wrote in message
...
Yuri Blanarovich wrote:
That is rather simplistic and not reality reflecting explanation.
Like saying that ground plane, vertical antenna is a dipole.
Vehicle body represents "ground plane" similar to two or more radials.
The current flows along the surface of the vehicle, just as along the
more elaborate ground plane consisting of more than say 8 radials.
Cancellation of current along the body happens, just like in opposite
radials in GP.
It appears that you missed my explanation, so I'll try again.
The currents flowing different directions don't cancel. If they flow in
equal amounts, in phase, in opposite directions, then the fields they
create nearly cancel. And that's the case along the roof of a car if the
car and roof are symmetrical and the antenna is at the center. But it's
not the case where it flows vertically along the sides of the car. There,
the currents are in the same direction.
Radiation pattern is formed between the RADIATOR (whip) and GROUND PLANE
(vehicle body).
The radiation pattern is formed by the sum of all the fields which are
created by currents flowing on conductors. The antenna is one such
conductor. The body of the car is another.
Additional effect is that vehicle "ground plane" is capacitively coupled
to the ground and this is reflected in changes in efficiency depending on
the surroundings ground conditions (salty, wet ground, reinforced
concrete bridges, etc.)
That's true. And the coupling of the car body to ground alters the amount
of current flowing along the body of the car. This current equals the
current flowing into the antenna.
As far as I understand, dipole refers to dual pole antenna with
symmetrical current distribution.
Suit yourself. I called the system an "asymmetrical dipole". But like
"ground", putting a name on it doesn't change its properties.
Vertical antenna mounted on conducting body of vehicle has current
distribution in the "other pole" far from symmetrical. This can be seen
in modeling in EZNEC. Try to compare vertical whip mounted on vehicle,
with dipole that has one leg horizontal and you will see the difference,
far from "nice dipole" antenna.
So as soon as we have more than one radial, and some (horizontal)
cancellation is happening it ain't no dipole. It is monopole forming
vertical pattern against the ground plane (radials, vehicle body).
Certainly a whip mounted on a vehicle can be expected to have a different
pattern than a symmetrical dipole, and nothing I've written has attempted
to make a claim that it does. But it sounds like you've grabbed onto the
"asymmetrical dipole" label as a basis for argument. So please go back
over my postings but substitute "Yuri special" for "asymmetrical dipole"
and see if then you find anything I've written which isn't correct.
That's the way I understand it, without involving photons, Gaussss and
other farticles :-)
If you say so.
Roy Lewallen, W7EL
|